Various neurotransmitters and their functions

July 5, 2018

Neurotransmitters are chemical agents synthesized by nerve cells.
Are stored in secretory vesicles in axon terminals.
Released when a nerve impulse reaches the end bulb.
Also defined as the chemicals which allow the transmission of signals from one neuron to the next across synapses.
Produce an excitatory or inhibitory response in the post synaptic membrane.
Released from synaptic vesicles in synapses into the synaptic cleft.
In synaptic cleft, they are received by neurotransmitter receptors on the target cells.
For example: acetylcholine evokes an excitatory response at the motor end plate in a skeletal muscle cell. whereas it evokes an inhibitory response at its synapse with a cardiac muscle cell.
Many possible neurotransmitters are being investigated.
There are four main criteria for identifying neurotransmitters:

The chemical must be synthesized in the neuron or otherwise be present in it.
When the neuron is active, the chemical must be released and produce a response in some target.
The same response must be obtained when the chemical is experimentally placed on the target.
A mechanism must exist for removing the chemical from its site of activation after its work is done.

About 50 are present in our nervous system and there may be over 100.
Known neurotransmitters are classified into four groups. They are:

1) Acetylcholine

It is the main neurotransmitter released by neurons of the peripheral nervous system.
Only a restricted number of neurons in the central nervous system release acetylcholine.
The acetylcholine released by lower motor neurons stimulates contraction of skeletal muscle.
In contrast, the acetylcholine released by the postganglionic neurons of the peripheral nervous system inhibits contraction of cardiac muscle.

2) The amino acids neurotransmitters

Amino acid neurotransmitters release (exocytosis) is dependent upon calcium Ca²⁺and is a presynaptic response.
There are inhibitory amino acids (IAA) or excitatory amino acids (EAA).
Some of them include gamma-aminobutyric acid (GABA), glycine, aspartic acid, and glutamic acid.
Although GABA is not an amino acid, it is classified with the amino acids because it is derived from glutamic acid.
It is the major inhibitory transmitter of the small local circuit neurons in such structures as the cerebral cortex, cerebellum, and upper brainstem.
Glycine, the simplest amino acid, is the major inhibitory transmitter of local circuit neurons in the lower brain stem and spinal cord.
It selectively inhibits certain neurons to refine and focus brain activity into meaningful patterns by eliminating and suppressing nonessential activity.
Glutamic acid and aspartic acid are excitatory transmitters in the central nervous system.

3) Monoamines

Monoamine neurotransmitters are neurotransmitters and neuromodulators.
Contain one amino group connected to an aromatic ring.
connection occurs by a two-carbon chain (such as -CH₂-CH₂-).
Include catecholamines, such as norepinephrine and dopamine, and indoleamines, such as serotonin.
Nor-ephinephrine (noradrenaline) is located in neurons with cell bodies in the brain stem and in post-ganglionic neurons of the sympathetic nervous system.
Norepinephrine releasing fibers are distributed widely in structures such as the cerebral cortex, cerebellum and spinal cord.
Fibers that terminate in the cerebral cortex are involved with various levels of consciousness.
Pep pills containing the drug amphetamine increase the level of norepinephrine in the brain by blocking its reuptake in the neuron and inhibiting the action of enzyme monoamine oxidase (MAO).
The euphoria and hallucinations produced by amphetamines are thought to result from the inhibition of MAO.
An amphetamine psychosis which is quite similar to schizophrenia can result from the habitual use of amphetamines.
Chlorpromazine is used in the treatment of schizophrenia and similar disturbances.
It acts by blocking the reuptake of norepinephrine.
Dopamine is neurotransmitter found in the area of brain where certain neural circuits are involved with voluntary motor integration.
Deficits of dopamine involving the basal ganglia are associated with Parkinson’s disease.
Serotonin is associated with various mood swings, including depression, elation, insomnia, and hallucinations.

4) Neuropeptides

Chains of neuroactive aminoacids.
Found in the central nervous system (CNS).
Somatostatin, the endorphins and the enkephalins are examples.
Release of growth hormone from the pituitary gland is acted upon by somatostatin.
Inhibitory function is shown by somatostatin to growth hormone.
Apparently, it acts by altering the excitability of the post synaptic membrane of target cells.
Endorphins (morphine like substances) and enkephalins (in the head substances) are naturally occurring peptides.
Found in several regions of the central nervous system.
Function at post synaptic receptor sites in the pain pathways.
Suppress synaptic activity leading to pain sensation.
These opiates are not used therapeutically because their effectiveness is relatively short-lived and because they are addictive.
Many neuropeptides are classified as neuromodulators.
They are the chemical agents that are capable of altering the responsiveness of neurons to a neurotransmitter.
Neuropeptides, histamines, prostaglandins, and the hormones cortisol and estrogen are thought to have neuromodulator functions.